1. Field of the Invention
The present invention relates to code division multiple access communications systems and related methods of operation. More particularly, the present invention relates to cellular communications systems and signal processing apparatus and methods employed in cellular communications systems.
2. Background of the Prior Art and Related Information
Wireless communications systems employing transmission between base stations and multiple mobile users are a key component of the modern communications infrastructure. (Such wireless communications systems are referred to herein as “cellular” communications systems for brevity and without limiting the term cellular to the specific types of communications systems or specific frequency bands to which the term is sometimes associated.) These cellular systems are being placed under increasing performance demands which are taxing the capability of available equipment, especially cellular base station equipment. These increasing performance demands are due to both the increasing number of users within a given cellular region as well as the bandwidth requirements for a given channel. The increasing number of cellular phone users is of course readily apparent and this trend is unlikely to slow due to the convenience of cellular phones. The second consideration is largely due to the increased types of functionality provided by cellular phone systems, such as Internet access and other forms of data transfer over the cellular phone system. These considerations have resulted in a need for more channels within the available spectrum provided to cellular phone carriers as well as more bandwidth for each channel.
The traditional approach to fitting as many channels as possible into an available frequency spectrum is to place each channel in a narrow frequency band. The individual channels must be sufficiently far apart in frequency to avoid significant interference between the individual cellular system users, however. Also, the narrower the frequency band for a given channel the less bandwidth which is available for the particular channel.
An alternative approach to providing the maximum number of channels in a given frequency spectrum, which has been adopted in more and more digital cellular systems, is code division multiple access spread spectrum communication. When digital information is transmitted from one location to another the data bits are converted to data symbols before transmission. The bandwidth of the transmitted signal is a function of the number of symbols transmitted per data bit sent. In code division multiple access spread spectrum communication, more symbols are transmitted than the data bits to be sent. In particular, for each data bit to be sent a multi symbol code is transmitted. The receiver, knowing the code, decodes the transmitted signal recovering the data bits sent. With a suitable choice of unique codes, many users can communicate in the same bandwidth without interference since each channel is orthogonal through coding. In code division multiple access spread spectrum cellular systems the spreading code is typically chosen to spread the data from an individual channel across a relatively wide frequency spectrum, within of course the spectrum range available to the given cellular provider. This minimizes interference between channels and maximizes the number of channels in the available frequency spectrum. Currently, two standards exist which relate to code division multiple access cellular communications systems. These standards are commonly known as CDMA and WCDMA for Code Division Multiple Access and Wide Code Division Multiple Access. Due to the highly effective use of the available frequency spectrum CDMA and WCDMA are increasingly being adopted as the solution of choice to accommodate increased cellular use.
A problem exists, however, with the practical implementation of spread spectrum cellular systems due to the manner in which the multiple user channels are combined to create the spread spectrum signal. This may be appreciated by referring to FIG. 1 which illustrates spread spectrum signal generation in a typical prior art cellular base station implementation. As shown in FIG. 1, in a spread spectrum system, a code-multiplexed signal generator 10 receives a plurality of data channels D, e.g., n in number, corresponding to the number of users which can be accommodated. A train of symbols is created for each communication channel by multiplying the input symbols for each channel by a separate orthogonal code. The amplitude of each channel may differ based on individual channel power needs. Each symbol train is then added to create a single code multiplexed symbol train (having in-phase and quadrature components, V1 and V2 in FIG. 1). The code multiplexed symbol train is then passed through a filter 20 to create the desired output signal. This filter plays a critical role since it imposes a “spectral mask” over the symbol train that ensures the broadcast signals stay within the spectrum allocated to the cellular carrier. Failure to observe such limitations on spectrum allocation can violate federal regulations as well as causing noise in neighboring bands of a given carrier. The output signal is then provided to a digital to analog converter 30 resulting in an analog signal that is mixed with a carrier signal in a modulator 40. The resulting RF signal is provided to an RF power amplifier 50 and broadcast to the cellular users.
The problem begins in the combining of the multiple symbol train in the code multiplexor 10 in FIG. 1. Since many individual symbol trains are combined, the peak power of the overall signal output from the filter will depend on the individual amplitudes of the symbols being combined. It is statistically possible that the individual channel symbols will add to create very large combined symbol peaks. Although statistically not common, such very large symbol peaks must be accommodated in the overall system design. Accommodating such large symbol peaks in the overall system creates practical implementation problems. For example, the presence of potentially very large peaks in the signal being output from the filter to the digital-to-analog converter requires a very high resolution digital-to-analog converter to be used. This adds cost and complexity to the overall system.
Another problem associated with potentially very large signal peaks in a code division multiple access spread spectrum system relates to the difficulty of providing linear amplification of the signal by the RF power amplifier. In cellular systems, it is very important to provide linear amplification of the broadcast signal. This is the case since non-linear amplification of the signal can result in distortion in the signal as well as creation of spectral sidebands that can interfere with other cellular frequency bands. Since cellular frequency bands are strictly regulated, cellular systems must be carefully designed so that such creation of noise outside of the allocated frequency band is avoided. Therefore, linear RF amplification is necessary in cellular base stations. To operate an amplifier in its linear range, however, requires that the amplifier be operated in a relatively low power mode. If large random peaks in the signal are to be accommodated by such an amplifier and still keep it operating in the linear regime, a higher power RF amplifier is required. High power, high quality RF amplifiers are very expensive and this thus adds significant cost to the overall base station system.
The problem of large random peaks in the signal is therefore a significant problem in the practical implementation of spread spectrum cellular communications systems.
The significance of the problem of large random signal peaks has been appreciated in the prior art and solutions to this problem have been attempted. For example, an approach to solving this problem is described in U.S. Pat. No. 6,009,090 to Oishi, et al. The approach of the '090 patent is illustrated in FIG. 2. A signal peak suppression unit 60 is placed in the signal generation path after the code multiplexor 10 which adds the individual symbol trains together. This signal peak suppression unit compares the multiplexed symbols to a maximum permitted value and then simply truncates those symbols that exceed that maximum permitted value. Although this peak suppression unit solves the problem of large symbols, it fails to remove all the large signal peaks that must be processed by the D/A converter and power amplifier. In addition, when a symbol is truncated, a less than ideal symbol is sent, which will increase communication errors. This may be appreciated by carefully considering the effect of the signal peak suppression unit on the symbols as they continue through the signal generation path.
As illustrated in FIG. 2, after the peak suppressed symbols leave the peak suppression unit, they pass through a filter 20. The filter 20 can be represented by an impulse response function. A typical spread spectrum impulse response function is shown in FIG. 3 (WCDMA, root raised cosine, α=0.22). The impulse response of the filter is impressed on each code multiplexed symbol as the symbols pass through the filter. This impression of the filter impulse response on the symbols can increase or decrease peaks at the on-symbol interval and can create new peaks between symbol times. More specifically, FIG. 4 shows how the filter output peaks can differ from the input symbol peaks. FIG. 4 displays the filter output caused by two consecutive input symbols of amplitude 1. The two input symbols produce the filter impulse response functions shown by the solid and dashed lines in FIG. 4 at the filter output. The true filter output would thus be the combination of these two responses (but this addition is not performed in FIG. 4 for ease of illustration). At symbol time 0, one impulse response is at its maximum and the other is slightly negative. The signal output will therefore be lower than the input symbol amplitude at symbol time 0, for this case. (If the second symbol had been negative instead of positive the signal would have been larger than the input symbol at symbol time 0.) The output signal will reach a maximum at symbol time 0.5 (inter-symbol) when the two filter responses add to produce a combined output of about 1.2.In an actual output signal, these effects will be enhanced by the influence of the additional symbols simultaneously present in the filter.
FIGS. 5A and 5B illustrate how a given input symbol and the symbols preceding and following that symbol in the symbol train can statistically create a range of output signal values as the symbols pass through the filter. FIGS. 5A and 5B are complex vector diagrams illustrating an input symbol as a vector from the origin of the complex plane (in-phase and quadrature signal components). FIG. 5A shows the input symbol slightly exceeding a desired peak limit value (illustrated by the dashed line). In FIG. 5B, the input symbol is precisely on the limit line. The filtered output signal is a function of the input symbols and the impulse response function of the filter. As is apparent from the discussion of FIG. 4, the output signal peaks will randomly differ from the input symbol peaks since the differences are caused by the filter response to random symbols preceding and following that symbol in time. This random effect is statistically represented in the figures by the solid circle labeled “predicted filter output”.
When the effect of the filter on the symbol train passing through the filter is considered, the result of the signal peak suppression unit of the above noted '090 patent is dramatically altered. For example, assuming the input symbol illustrated in FIG. 5A the '090 patent would always peak suppress this symbol as it exceeds the limit value and thus always introduce some distortion by this process. The actual value which is D/A converted and RF amplified, however, is the filtered output which statistically is represented by the circle. As may be seen, some of the time this filtered value will be inside the limit value and not require limiting. On the other hand, some of the time the filtered value will exceed the limit by more than the input symbol and will not be adequately peak adjusted even if the input symbol is truncated to the limit value. In the example of FIG. 5B in turn, the input symbol does not exceed the limit value and in the approach of the '090 patent all such symbols would pass through unaffected. As may be appreciated from the circle of filtered outputs in FIG. 5B, however, the effect of the filter is that output signals will actually exceed the limit value significantly. Therefore, for this situation the signal peak problem would not be solved by the approach of the '090 patent unit at all. Therefore not only does the approach of the above noted '090 patent introduce unnecessary distortion into the signal where peak reduction is not necessary, it also completely fails to eliminate many of the excessive peaks in the output signal, the very problem it was designed to solve.
Although not discussed in the above noted patent, an alternative approach might be to simply place the peak suppression unit on the downstream side of the filter 20 shown in FIG. 2. This also introduces a problem, however, since the presence of the peak suppression unit will inevitably distort the filter output signal. This will create spectral noise that extends beyond the spectral mask the filter was designed to maintain. As noted above, the spectral mask created by the filtering of the signal is critical in cellular systems since exceeding spectral allocations can potentially violate federal regulations.
Therefore, whether the peak suppression unit is placed before the filter or after the filter it is clear that such a solution is completely inadequate to solve the problem of large peaks in the output signal and such solution either fails to eliminate the peaks or introduces equally significant problems. Therefore, such an approach is unworkable in real world applications.
Accordingly, it will be appreciated that a need presently exists for a system and method of providing code division multiple access spread spectrum cellular transmission which avoids the above noted problem of large signal peaks and accompanying constraints and costs associated with the RF amplification and digital-to-analog conversion of such large peaks. Furthermore, it will be appreciated that a need exists for such a system and method which does not introduce significant additional new problems to the system and which can be implemented without undue cost or other complexities of implementation.